Parallel Digital Coherent Detection Using Symmetrical Optical Interleaver and Direct Optical Down Conversion

ABSTRACT

A method includes modulating lightwaves to provide first and second OFDM signal sidebands at a first polarization direction and first and second OFDM signal sidebands at a second polarization direction, and combining sidebands that are oppositely positioned and joined from the first and second OFDM signal sidebands at each polarization direction to provide a polarization multiplexing OFDM signal.

This application claims the benefit of U.S. Provisional Application No. 61/169,361, entitled “Parallel Digital Coherent Detection Using Symmetrical Optical Interleaver And Direct Optical Down Conversion”, filed on Apr. 15, 2010, the contents of which is incorporated by reference herein.

BACKGROUND OF THE INVENTION

In high-capacity, long-haul fiber data transmission, digital signal processing (DSP) after signal detection can greatly improve transmission performance by providing compensation against fiber impairments such as chromatic dispersion (CD) and polarization mode dispersion (PMD). Together with polarization diversity coherent detection, DSP also grants polarization division multiplexing (PDM) to double the transmitted data rate. To implement DSP for post detection processing, analog-to-digital converters (ADCs) at high sampling rate are needed in the coherent receiver design. In reality, the signal bandwidth that can be supported by electronic ADC is much lower than the E-O modulator bandwidth at the transmitter. Currently, the state of the art is about 20 GHz for electronic ADC bandwidth, while 40 GHz bandwidth E-O modulators are already commercially available. To fully utilize the potential of post-detection DSP at high data rates, a new ADC technology is required to fill the bandwidth gap.

Parallel processing of incoming high BW signals is often used to achieve high ADC sampling rates. Electronic time-interleaved ADCs have synchronized track-and-hold circuitry, which has to cover the entire input signal BW (state of the art ˜20 GHz), on each parallel sampling path. Parallel processing in frequency domain, which can reduce the BW requirement of each sampling path, has been proposed to reach higher sampling rates. We proposed recently a photonic filter bank (PFB) structure using orthogonal filter design to allow digital perfect reconstruction in theory. In order to sample the high frequency tributary of the incoming signal, filter bank methods need RF electronic frequency down converters. The need for rf down conversion not only increases system complexity and cost (a typical phase/polarization diversity receiver requires a total of four converters), but it may also degrade system performance because of the difficulty of analog wideband processing in electronic domain. In terms of filter design, it is desirable to have sharp filter roll-offs because of the limited ADC BW in each parallel sampling paths. In PFB structure, sharp roll-offs are challenging to implement using orthogonal filter design because many optical taps are required.

Accordingly, there is need for a method to generate over 100 Gbit/s OFDM signals with the limited bandwidth for A/D and D/A converter tolerance.

SUMMARY OF THE INVENTION

In accordance with the invention, a method includes modulating lightwaves to provide first and second OFDM signal sidebands at a first polarization direction and first and second OFDM signal sidebands at a second polarization direction, and combining sidebands that are oppositely positioned and joined from the first and second OFDM signal sidebands at each polarization direction to provide a polarization multiplexing OFDM signal.

In another aspect of the invention, an apparatus includes a modulator for varying lightwaves to provide first and second OFDM signal sidebands at a first polarization direction and first and second OFDM signal sidebands at a second polarization direction; and a polarization beam combiner for combining sidebands that are oppositely positioned and joined from the first and second OFDM signal sidebands at each polarization direction to provide a polarization multiplexing OFDM signal.

BRIEF DESCRIPTION OF DRAWINGS

These and other advantages of the invention will be apparent to those of ordinary skill in the art by reference to the following detailed description and the accompanying figures.

FIG. 1 is a diagram illustrating a PFB architecture with two parallel ADC sampling paths, in accordance with the invention

FIG. 2 is a diagram illustrating details of the front end of the digital coherent scheme, in accordance with the invention.

FIG. 3 is a diagram of transformations of an optical signal by the PFB architecture, in accordance with the invention.

DETAILED DESCRIPTION

The invention is directed to an inventive PFB architecture that uses an optical interleaver as a two-way filter bank. The optical interleaver exhibits a symmetrical spectral response about the optical carrier frequency, which will automatically translate the two filtered optical signal to low-frequency and high-frequency tributaries after O-E conversion. Optical interleavers can be easily designed to have very sharp roll-offs so that the filtered tributaries will fit into the sampling BW of electronic ADC. Another aspect of the PFB architecture is a direct optical down conversion scheme, which uses two synchronized optical local oscillators (LO) during coherent detection located with respect to the high-frequency tributary in such a way that RF down converters are no longer required. With this new invention, we can nearly double the supported signal BW for digital coherent detection.

FIG. 3 is a diagram of an exemplary 100 Gbit/s OFDM optical signal generation for transmission, in accordance with the invention, with only one RF frequency. FIG. 4 is a diagram of an exemplary reception of 100 Gbit/s OFDM optical signal generated for transmission in accordance with the invention. FIG. 5 a diagram of a modification to the configuration of FIG. 3 to show 100 Gbit/s OFDM optical signal generation with two RF frequencies for transmission, in accordance with the invention.

The diagrams of FIGS. 3, 4 and 5 are exemplary configurations using the following optical and electrical components: lightwave source 301, 501, RF frequency 304, 505, 506; electrical mixer 303, 304, 504; optical coupler 306, 507; intensity modulator 302, 502; optical filter 305, 508; and optical polarization beam combiner 307, 509.

The lightwave 301, 501 can be a narrow linewidth laser less than 2 MHz and the intensity modulator generates optical carrier suppression signals. The electrical mixer 303, 304, 504 up-converts the baseband signal to an RF band. The RF signal 304, 505, 506 is provided to the electrical mixer so that the base-band can be up-converted. The optical filter 305, 508 is realized by an optical interleaver so that only a high or low frequency signal can be passed for each port if the interleaver has two ports. Preferably, the interleaver has two input ports and one output port with sharp edge characteristics. The optical coupler 306, 403, 507 are preferably 50% to 50% ratio optical couplers that divide the signal into two equal parts. The optical beam combiner or splitter 307, 404, 509 combines or splits the orthogonal signal. The electrical combiner 503 combines two different frequency RF signals.

Referring to the diagram of FIG. 3, each intensity modulator 302 is driven by the mixed OFDM signal at RF frequency of f 304 by an electrical mixer 303. The lightwave 301 is split into two parts by an optical coupler 306. Then the two parts will be split again by the same optical coupler 306. There are two polarization directions. We assume that the up-subchannel is X polarization direction and the bottom-one is Y polarization direction. Each modulator 302 is operated at carrier suppression OCS mode. After the modulator, the carrier is suppressed. Then for each polarization direction, we use an optical filter 305, such as an optical interleaver to combine the two subchannels. When the interleaver 305 is matched to the wavelength of the input lightwave, we can generate an optical spectrum 308 and 309 as shown in FIG. 3. Each one just passes through half of spectrum (right or left). The optical filter 305 plays a key role tin generating the optical spectrum 308 or 309 and this is the main difference from that technique of FIG. 1 or 2. For example, in this figure with the invention, only right (black) and blue (left) can pass the interleaver. Then both sidebands can be used to carry the optical signals. After combing the X and Y polarization direction subchannels by an optical polarization beam combiner 307, we can generate polarization multiplexing OFDM optical signals.

The diagram of FIG. 4 shows an exemplary receiver configuration for receiving the 100 Gbit/s OFDM signal generated according to FIG. 3. The incoming lightwave is separated into two parts by an optical filter 401, interleaver or other optical filter. Then the right and left side will be detected by a regular 90 degree polarization-diversity coherent detector which includes a local oscillator LO 402 fed through optical couplers 403, 404 to separate coherent detectors 403.

The OFDM signal is generated from the D/A converter. Due to the D/A converter bandwidth limitation, the OFDM signal may not be high enough to carry a signal for over 100 Gbit/s signal (the total capacity with all sub-channels). So we need to change FIG. 3 to FIG. 5 to add one more RF frequency. Here, two RF frequencies, f1 505 and f2 506 are used. They are used to carry the OFDM signal and drive the modulator. The overall architecture is similar to FIG. 3, only one more RF frequency is used. From FIG. 5 we can see that more spectrum components are generated.

The present invention has been shown and described in what are considered to be the most practical and preferred embodiments. It is anticipated, however, that departures may be made therefrom and that obvious modifications will be implemented by those skilled in the art. It will be appreciated that those skilled in the art will be able to devise numerous arrangements and variations which, not explicitly shown or described herein, embody the principles of the invention and are within their spirit and scope. 

1. A method comprising the steps of: modulating lightwaves to provide first and second OFDM signal sidebands at a first polarization direction and first and second OFDM signal sidebands at a second polarization direction; and combining sidebands that are oppositely positioned and joined from the first and second OFDM signal sidebands at each polarization direction to provide a polarization multiplexing OFDM signal.
 2. The method of claim 1, wherein the polarization multiplexing OFDM signal has a bit rate of at least 100 Gbit/s.
 3. The method of claim 1, wherein the OFDM signal sidebands comprise oppositely positioned single sidebands about an RF frequency f, frequency f1 being a frequency to which data signals are upconverted to before driving the modulating of the lightwaves.
 4. The method of claim 1, wherein the OFDM signal sidebands comprise a first oppositely positioned sideband pair centered about an RF frequency f1 and a second oppositely positioned sideband pair centered about an RF frequency f2, frequency f1 and frequency f2 being frequencies to which respective data signals are upconverted to before driving the modulating of the lightwaves.
 5. The method of claim 1, wherein the OFDM sidebands are oppositely positioned and joined by filtering together for each polarization direction a sideband from the first OFDM signal sidebands with an oppositely positioned sideband from the second OFDM signal sidebands.
 6. The method of claim 2, wherein the step of combining comprises filtering together oppositely positioned sidebands from the first and second OFDM signal sidebands at each polarization direction.
 7. The method of claim 3, wherein the filtering comprises interleaving the oppositely positioned sidebands from the first and second OFDM signal sidebands at each polarization direction.
 8. The method of claim 1, wherein the step of combining comprises an optical polarization beam combiner for combining the opposed and joined sidebands.
 9. The method of claim 1, wherein the step of modulating is preceded by splitting a lightwave into first and second lightwaves at a first polarization direction and first and second lightwaves at a second polarization direction
 10. The method of claim 1, wherein the modulating comprises intensity modulations in carrier suppression mode and driven by respective mixed OFDM signals at an RF frequency f.
 11. The method of claim 1, wherein the modulating comprises intensity modulations in carrier suppression mode and driven by respective mixed OFDM signals at an RF frequency f1 and respective mixed OFDM signals at RF frequency f2.
 12. An apparatus comprising, a modulator for varying lightwaves to provide first and second OFDM signal sidebands at a first polarization direction and first and second OFDM signal sidebands at a second polarization direction, and a polarization beam combiner for combining sidebands that are oppositely positioned and joined from the first and second OFDM signal sidebands at each polarization direction to provide a polarization multiplexing OFDM signal.
 13. The apparatus of claim 12, wherein the polarization multiplexing OFDM signal has a bit rate of at least 100 Gbit/s.
 14. The apparatus of claim 12, wherein the OFDM signal sidebands comprise oppositely positioned single sidebands about an RF frequency f.
 15. The apparatus of claim 12, wherein the OFDM signal sidebands comprise a first oppositely positioned sideband pair centered about an RF frequency f1 and a second oppositely positioned sideband pair centered about an RF frequency f2, frequency f1.
 16. The apparatus of claim 1, wherein the OFDM sidebands are oppositely positioned and joined by interleaving together for each polarization direction a sideband from the first OFDM signal sidebands with an oppositely positioned sideband from the second OFDM signal sidebands.
 17. The apparatus of claim 1, wherein the modulator comprises optical carrier suppression to provide the modulated OFDM signal sidebands.
 18. The apparatus of claim 1, wherein the modulator comprises intensity modulations in carrier suppression mode and driven by respective mixed OFDM signals at an RF frequency f.
 19. The apparatus of claim 1, wherein the modulator comprises intensity modulations in carrier suppression mode and driven by respective mixed OFDM signals at an RF frequency f1 and respective mixed OFDM signals at RF frequency f2. 